This invention relates to a process for the stabilization of certain poly(ether ketone ketones), which contain units derived from diphenyl ether as well as 1,4-benzenedicarbonyl and 1,3-benzenedicarbonyl units.
Poly(ether ketone ketones), hereinafter sometimes referred to as PEKK's, especially those having the above chemical structure, are well known engineering polymers, which find wide use in certain high value applications such as, for example, in fabricating panels for aircraft interiors. They have high melting points, yet are melt processable and are reasonably well resistant to ignition.
PEKK's are made industrially by Friedel-Crafts catalyzed condensation of diphenyl ether with terephthalyl chloride and isophthalyl chloride in an inert solvent such as, e.g., o-dichlorobenzene, nitromethane, or ethylene chloride. The most commonly used catalyst is aluminum chloride, which always is employed in a large excess. The so-produced PEKK's usually have xanthydrol end groups which result from acylation in the ortho position of diphenyl ether. Those xanthydrol groups are thermally unstable and decompose on heating with the liberation of xanthone and formation of a phenyl free radical, which reacts with the polymer, causing crosslinking and degradation by hydrogen abstraction.
This thermal degradation is illustrated below in the following equation: ##STR1##
While this elimination of xanthone can be retarded by heating the polymer with formic acid, and thus reducing the xanthydrol groups to xanthene groups, exposure of the polymer to oxygen regenerates xanthydrol, which then rearranges to xanthone. Thus, the formic acid treatment simply slows down the degradation process but does not eliminate it.
It thus is very important to be able to permanently thermally stabilize PEKK's.